1. Field of the Invention
The present invention relates to a method of producing a liquid crystal display unit. More particularly, the present invention relates to a method for producing a liquid crystal display unit, in which a series of process of from a substrate bonding step to a liquid crystal injection and sealing step is simplified.
The present application is based on Japanese Patent Application No. Hei. 9-277330, which is incorporated herein by reference.
2. Description of the Related Art
Generally, a liquid crystal display unit is produced by a process comprising steps of: applying an orienting agent onto a pair of initially cleansed glass or plastic substrates; baking the substrates; after the step of baking, applying a rubbing treatment to the substrates; applying a sealing agent onto the substrates to bond the substrates to each other; injecting a liquid crystal into between the substrates; and, after the step of injecting, sealing a liquid crystal injection hole.
For bonding the substrates to each other, first, a spacing material for keeping the gap between the substrates constant is sprayed. The spacing material spray method is classified into a wet type and a dry type. Conventionally, the wet type in which a mixture of a spacing material and a liquid such as flon or alcohol is sprayed, has been the main current. However, the dry type spray method is advantageous in evenness of spacing material spray density, absence of spacing material lumps, etc., and therefore has begun to be established in recent years.
After a spacing material is sprayed, a UV-curing adhesive agent is applied onto a peripheral portion of a lower substrate. In the inverted or turned-over state, an upper substrate is bonded to the lower substrate. After pre-alignment and accurate alignment, the UV-curing adhesive agent is hardened to thereby temporarily bond the substrates to each other. Then, the substrates are heated to perform hardening and regular bonding at a temperature in a range of from 150 to 200.degree. C. for several hours under the pressure of about 1000 kg (maximum) by a hot-pressing method. Thus, cells are formed.
For injection of a liquid crystal into each cell, first, the inside of the cell is vacuum-deaerated through a liquid crystal injection hole and, at the same time, the inside of a liquid crystal reservoir is deaerated. In this case, the injection hole is small and narrow so that it is difficult to deaerate the cell. Therefore, in order to improve the deaeration effect, for example, evacuation is performed for a long time, and further, the cell is heated. In this case, when deaeration is performed while the liquid crystal is more or less cooled to increase the viscosity of the liquid crystal to thereby suppress the vaporization of the liquid crystal and stirred, the deaeration effect is relatively enhanced.
Then, an evacuation pump is stopped and the cell and the liquid crystal reservoir are connected to each other so that a liquid crystal material is injected into the cell by use of capillarity of the cell. When a large part of the cell is filled with the liquid crystal, dried inert gas is introduced into the liquid crystal reservoir to press the liquid crystal reservoir. By the pressure, injection is continued to fill the cell with the liquid crystal.
Since the viscosity of the liquid crystal is generally high, it is difficult to inject the liquid crystal into the cell. If the liquid crystal material is forcedly injected, the surface of an oriented layer near the injection hole may be injured. When the cell and the liquid crystal reservoir are heated to reduce the viscosity of the liquid crystal after completion of vacuum-deaeration in order to avoid the aforementioned disadvantage, the injecting work is easily performed, and further, the surface of the oriented layer is prevented from being injured.
Afterwards, the injection hole of the cell filled with the liquid crystal is sealed. In this case, the periphery of the injection hole is wet with the liquid crystal. Accordingly, it is very difficult to wipe the liquid crystal only on the periphery of the injection hole to prevent air bubbles from being mixed in the liquid crystal. In order to seal the wet injection hole, there were conventionally used various methods such as a method in which the injection hole was vapor-deposition-coated or plated with a metal such as chrome, nickel, copper, gold, or- the like, so that the injection hole wet with the liquid crystal was sealed with solder, a method in which a wood's metal such as iridium, tin, lead, or the like, was inserted into the injection hole to temporarily seal the injection hole and then the injection hole was sealed with an adhesive agent after the liquid crystal was wiped, etc.
In the aforementioned conventional method for producing a liquid crystal display unit, however, the process of bonding the substrates to each other and the process of injecting the liquid crystal were separated independently. Accordingly, there was a drawback in that independent production lines for the two processes were required and the equipmental scale became large. Furthermore, troublesome operation and high-grade technique were required for the substrate bonding work and the liquid crystal injecting work.
That is, since the spacing material was disposed with uniform density without any lamp, a high-grade spray technique was required.
In order to make the electrodes of rear and front substrates coincide with each other, pre-alignment and accurate alignment had to be performed stepwise when the substrates were bonded to each other.
Since the inside of the cell had to be vacuum-deaerated securely in spite of the small injection hole, it was necessary to evacuate the cell for a long time and heat the cell to thereby enhance the deaeration effect. Further, in order to enhance the deaeration effect, it was necessary to perform vacuum-deaeration while more or less cooling the liquid crystal to increase the viscosity of the liquid crystal and stirring the liquid crystal.
On the other hand, in order to fill the cell with such a high-viscosity liquid crystal securely, it was necessary to heat both the cell and the liquid crystal reservoir after completion of vacuum-deaeration to thereby reduce the viscosity of the liquid crystal.
Furthermore, in order to seal the wet injection hole securely, there was required a method, or the like, in which the injection hole was vapor-deposition-coated or plated with a metal such as chrome, or the like, in advance so that the injection hole was sealed with solder after injection of the liquid crystal.
Accordingly, the conventional producing method had a drawback in that a long time was required for production and the cost of the product increased since equipmental cost increased and a troublesome producing process was required. Further, since the liquid crystal material was injected by use of capillarity, there was a drawback in that it was difficult to reduce the thickness of the resulting product and increase the area of the resulting product.